1. Field of the Invention
The present invention relates to a reference power supply which supplies a reference voltage to a load connected to the reference power supply.
2. Description of the Related Art
Electronic devices, such as portable telephones, incorporate a reference power supply which generates a reference voltage based on the voltage supplied from an external power supply. With the ever increasing trend to miniaturize such products, there is an accompanying demand for power supplies capable of delivering a lower reference voltage supply.
FIG. 1 illustrates a conventional reference power supply frequently found in electronic devices. The reference power supply comprises a reference voltage source 21 and an amplifier 22. The reference voltage source 21, formed from a bandgap reference circuit, supplies a given voltage V.sub.IN to the non-inverting input terminal of the amplifier 22. Based on the input voltage V.sub.IN, the amplifier 22 produces a reference voltage V.sub.REF supplied to a load (not shown). The reference voltage V.sub.REF is feed back to the inverting input terminal of the amplifier 22 via an external circuit consisting of resistors R11 and R12. That is, the amplifier 22 is a non-inverting amplifier circuit. Consequently, the non-inverting and inverting input terminals of the amplifier are considered to form an imaginary short-circuit, and therefore, share the same voltage potential. As a result, the voltage fed back to the inverting input terminal is obtained by dividing the reference voltage V.sub.REF by the resistors R11 and R12.
FIG. 2 shows the detailed structure of the amplifier 22. PNP transistors Q21 and Q22 have their emitters connected together to form a differential circuit. PNP transistors Q23, Q24 and Q29 have emitters connected to a power supply V.sub.CC, and bases connected together. Those transistors Q23, Q24 and Q29 forms current mirror circuits. The collector of the transistor Q23 is grounded via a constant current circuit 24. The transistor Q24 has a collector connected to the emitters of the transistors Q21 and Q22, allowing current to flow through the transistors Q21 and Q22 via their emitters.
NPN transistors Q25 and Q26 have bases connected together to form a current mirror circuit. The collector of the transistor Q25 is connected to the collector of the transistor Q21. The transistor Q26 has a collector connected to its own base and to the collector of the transistor Q22. NPN transistors Q27 and Q28 are connected in such a way as to form a Darlington circuit. The base of the transistor Q27 is connected to the collector of the transistor Q25. The transistor Q29 has a collector connected to an output terminal 23, with the resistors R12 and R11 connected in series being between the output terminal 23 and the ground.
The transistor Q29 supplies currents to the transistor Q27 and the resistor R12. The collector current of the transistor Q29 is supplied via the output terminal 23 to the load in order to drive the load. The level of the load driving current of the transistor Q29 is determined by the ratio of the emitter area of the transistor Q29 to that of the transistor Q23. The base of the transistor Q22 is connected to a node between the resistors R11 and R12. The emitters of the transistors Q25, Q26 and Q28 and one end of the resistor R11 are grounded.
In the conventional amplifier 22, the load driving current should be generated by the current mirror circuit composed of the transistors Q23 and Q29. This requires that the current mirror ratio (i.e., the ratio of the emitter area of the transistor Q29 to that of the transistor Q23) be set sufficiently large, and that the transistor Q29 have a large emitter area. The greater the current mirror ratio is set, the greater the base current of the transistor Q29 becomes. If the current mirror ratio is set too large, however, the desired current level will not flow through the transistor Q29.
This shortcoming can be overcome by a reference power supply circuit as shown in FIG. 3. This reference power supply has a reference voltage source 21 and an amplifier 25 having an additional NPN transistor Q30. The NPN transistor Q30 has a collector connected to a power supply V.sub.CC, an emitter connected to the output terminal 23, and a base connected to the collector of the transistor Q29. The transistor Q30 produces the load driving current in accordance with the base current supplied from the transistor Q29. This circuit structure reduces the necessity to increase the emitter area of the transistor Q29, and thus allows for the use of a smaller current mirror ratio.
Given that V.sub.BE is the voltage between the base and emitter of the transistor Q30 and that V.sub.CE is the voltage between the collector and emitter of the transistor Q29, the minimum voltage V.sub.CCMIN of the power supply V.sub.CC that can be used in the amplifier 25 satisfies the following equation. EQU V.sub.CCMIN =V.sub.REF +V.sub.BE +V.sub.CE
In other words, the voltage of the power supply V.sub.CC cannot be set lower than this minimum voltage. Thus, for example, with the reference voltage V.sub.REF set to 1.8 V, the base-emitter voltage V.sub.BE set to 0.7 V and the collector-emitter voltage V.sub.CE set to 0.1 V, the voltage of the power supply V.sub.CC should be set equal to or greater than 2.6 V.